Summary: The mammary gland is organized into ducts composed of a polarized epithelium that is in intimate contact on its basal surface with the extracellular matrix (ECM). This contact with the ECM is necessary to maintain the differentiated phenotype and tissue specific gene expression of mammary cells. This normal polarized organization is lost when mammary cells are transformed into invasive carcinoma. Associated with transformation is a loss of appropriate contact with the ECM, changes in various integrin subunits, a loss of the differentiated phenotype, and a switch to a more motile, invasive phenotype. A decrease in alpha2beta1 levels on mammary cells induces a more motile phenotype, and is correlated with more invasive, metastatic breast cancer, suggesting that signals transduced by the alpha2beta1 integrin are important to maintain normal cellular differentiation. Because of their effects on actin organization, Rho family members, Rho, Rac, and Cdc42, may mediate or modigy the effects integrins have on cell phenotype. Additionally, these molecules are members of the Ras superfamily, whose prototype member, Ras, is important in cellular transformation. Little is known about how these molecules affect integrin-mediated events, or whether they can induce a more transformed phenotype. We have found that altering Cdc42 in mammary epithelial cells disrupts alpha2beta1 integrin-mediated epithelial polarization and converts the cells to a motile, invasive phenotype, suggesting that activation of Cdc42, like Ras, might be part of both normal cellular signaling events and cellular transformation. Our preliminary results suggest as our hypothesis that activation of Cdc42 switches how mammary epithelial cells respond to collagen from a polarized, differentiated response to a motile, invasive response by affecting collagen-induced signaling pathways. Since Cdc42 has dramatic effects on cellular responses to collagen, the approach will be to use current knowledge about Cdc42 as an entry point into looking at potential signaling pathways that might be part of integrin-mediated effects on cell phenotype. This proposal is aimed at investigating both the transforming effect of Cdc42, as well as its role normal cellular signaling events.